This invention relates generally to a digital position sensor, and, more particularly to one of absolute position, and which is suitable for use in sensor bearings for automobile or industrial applications. A digital sensor of this type can be mounted in the bearings of an automobile steering column in order to check the path of the vehicle, or in systems for checking or controlling the position of a robot or of a machine tool, for example.
An absolute position sensor indicates very precisely the position of the moving components, so that they can be controlled and, above all, so that these components can be relocated when the system in which they are integrated is activated. Such sensors have the advantage of avoiding preliminary initialization before functioning, by detecting a reference pulse as in the case of a relative position sensor. An absolute position sensor delivers a number of output signals in the case of a parallel digital output signal, but in the case of a series digital output signal, the sensor delivers a single signal resulting from a shaping according to a data transmission protocol and executed from the signals in parallel described in therein.
Although the code provided by the output signals for marking the absolute position can be any code, a binary code or the Gray code is nevertheless preferably used. FIG. 1 is an example of parallel output signals encoded in the Gray code and delivering 512 points, in other words 512 different positions per revolution. Moreover, in the case of a sensor delivering a positive whole number n of output signals in parallel, there are generally n different tracks supported by the encoder, each having a specific coding and n stationary sensors intended for reading the information provided by these n tracks. In other words, the number of sensors and of coded tracks is equal to the number of available output signals allowing one to obtain a maximum resolution equal to 2.sup.n.
This type of device can be produced with an optical encoder, using a disk engraving technology allowing one to produce very fine lines. The advantage of such an optical encoder is that it delivers digital signals which are much more reliable than the signals coming from analog absolute position sensors of the potentiometric type, particularly when the ambient temperature varies. However, the use of such a device is limited to applications executed in ambient conditions which are not very severe, with a low level of pollution and vibrations, and at an ambient temperature less than 100 degrees C.
In order to reduce these limitations, there are magnetic encoders consisting of a multitrack and multipolar magnetic disk connected to the moving part of the device and several magnetic sensors connected to the stationary part of the device. On the moving disk, a number of magnetic pole pairs provide a sequence of equally distributed north and south poles, which successively pass before sensors of the type of Hall effect probes, magnetoresistive probes or of the inductive type. These magnetic position sensors can be advantageously integrated in bearings, as described in the French Patent Applications published under Nos. FR 2 678 329 and FR 2 660 028 in the name of the applicant.
In contrast, the main disadvantage of such magnetic encoders is the encumbrance of their large spatial requirements when several magnetic tracks are arranged on the disk, for the purpose of obtaining a coding and output signals identical to those obtained with an absolute position optical encoder. In effect, a minimum spacing must be kept between each of them in order to reduce their mutual influence, which is detrimental to the precision of the output signals representing the absolute position of the system in which the sensor is placed. Moreover, the traditional multipolar magnetization, obtained from a coiled magnetizer through which strong electric currents flow, is very complicated.
There are magnetic devices, however, which allow one to solve these problems of spatial requirements, on one hand, and of magnetization, on the other hand, consisting of a magnetic disk made of a thin layer material in which the technology of the magnetic writing allows for the writing of an extremely high density of information, and of inductive or magnetoresistive sensors suitable for reading the coded information written on the disk. Their disadvantages are of two types: they require a very small reading air gap, which is incompatible with conditions of industrial applications, and they are relatively fragile in use.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.